It's already free and always was supposed to be; iIn fact, you can play it for free RIGHT NOW by downloading it from the official website. Since it's an unofficial remake of a game made by another company, they can't legally charge money for it or even make donations, but they've stated that they're completely fine with that and are content knowing they've created something that people love.

The devs cannot charge without Valve's say so. The characters, plot, game engine, etc etc all belongs to Valve. It is just a mod, using Valve's work. That said, they worked really hard on it and that work is fantastic, but they knew from day one that they wouldn't be able to charge for it because they don't own the rights to do so.

If something happens and the game is pay for one day, that will be at Valve's discression only, and most likely, 90% or more of that would go straight to Valve.

I doubt however that it will cost anything ever since most of the game is already released for free on the developer's website.